R: [amsat-bb] strange orbit

["i8cvs" <domenico.i8cvs@xxxxxx>]

Hi Antonio

What make interesting the "elliptical geosyncronous" orbit of Sirius-1
for example is the designe of the orbit from the celestial point of view
because the selected inclination E0= 63.19020° is very close to the
critical inclination of 63.43° that stop the revolution of the line of
the apsides so that dW i.e. the rate of change of the argument of perigee
for Sirius-1 is only dW= + 0.000133°/day and so the latitude of the
apogee subpoint fall always over 63.2° North latitude for many years.

In addition the selected combination of  the above inclination with the
eccentricity E0= 0.26589530° and mean motion N0= 1.00271268
orbits /day make insignificant dRA i.e. the rate of change of the right
ascension of the ascending node and for Sirius-1 dRA= - 0.007°/day
so that the longitude of the ascending node over the equator happens
always at a crossing of 64.8° W longitude for many years.

For every orbit the apogee fall over 63.2° North latitude and the perigee
fall over - 63.2° South latitude for every orbit.

Sirius-1 is in elliptical orbit (E0=0.2658953) so that the orbital speed
at apogee is smaller than at perigee while the speed of rotation of the
earth spinning inside the orbital plane is constant and this is why the
resultant of the above speeds make the ground track of the satellite
subpoint of Sirius satellites to twist and resemble to a  "figure 8"

All of this can be recognized very well going deeply in to your program
InstantTrack but only after carefully reading your very informative file
IT manual.

Best 73" de

i8CVS Domenico


----- Original Message -----
From: Franklin Antonio <antonio@qualcomm.com>
To: G8IFF/KC8NHF <nigel@ngunn.net>
Cc: <amsat-bb@amsat.org>
Sent: Saturday, November 20, 2004 9:50 AM
Subject: Re: [amsat-bb] strange orbit


> At 03:00 PM 11/19/2004, G8IFF/KC8NHF wrote:
> >Would someone care to explain a figure of eight geo-synchronous orbit
> >around the earth that doesn't crash twice every orbit?
>
> The explanation is that the shape of the orbits of the Sirius satellites
> are elliptical, not "figure 8".  I know they keep calling them "figure 8"
> but that doesn't mean that the shape of the orbit is a figure 8.
>
> Most communication satellites are in circular orbits, because only
circular
> orbits can be geostationary (which means stays at the same place relative

> to the surface of the earth).  Unfortunately these circular geostationary
> orbits only work at 0 degrees inclination, meaning they orbit around the
> equator.  For folks on the equator that's great, but for folks at high
> latitudes the darn things appear at low elevations, ie down near the
> horizon which can be problematic.
>
> Now if you live at a high latitude, like the Russians, you might look for
a
> different solution.  If you're building a mobile satellite system where
you
> want cars to have a good chance of having a clear path to the satellite,
> not blocked by buildings, you might look for a different solution.
>
> A different solution is an elliptical orbit, which is tilted at a nice
high
> inclination, so that when the satellite is the farthest from earth (at
> apogee) and therefore going around the center of the earth the slowest, it
> will be sitting above some nice high latitude (like where you live)
instead
> of over the equator.  Because satellites in elliptical orbits spend most
of
> their time near the apogee, the thing will appear to hover there when
> viewed from earth, meaning it is usable a lot of the time.  Maybe not all
> the time, but a lot of the time.  If you have more than one of these
> satellites you can get them timed so that one or the other of them seems
to
> be hanging above you all the time.  A lot like juggling.  This is just
> about as good as a geostationary satellite, but obviously has some
> different properties.  Its better for people with higher latitudes.
That's
> good.  Geostationary satellites appear to stand still from the viewpoint
of
> a human on the surface of the rotating earth, but  elliptical orbit
> satellites appear to move.  That might be bad for providing TV to homes,
> for example, because people might need their dishes to move to stay
pointed
> at darn satellites.  If you are building a system for which people are
> gonna use omnidirectional antennas which don't need pointing (on their
cars
> for example) then you don't care that the satellite appears to move from
> the viewpoint of the human standing on earth.
>
> So if you look up at this geosynchronous elliptical orbit satellite from a
> fixed point on the rotating earth, what do you see?  If your eyes were
good
> enough you'd see the satellite move around a figure eight kinda
> thing.  That's why they call 'em figure eight orbits.  I don't call 'em
> that, but you gotta admit "figure 8" is probably a better term to put in
> marketing literature than "elliptical geosynchronous".
>
> Interestingly, real figure 8 shaped orbits are possible, but you can't
make
> 'em by only orbiting around one thing.  The apollo missions used 'em to go
> to the moon, so that they would head back toward earth automatically if
for
> some reason they couldn't slow down when they were around the moon.  I
used
> to fly figure 8 orbits around the sun and an orbiting planet in a space
war
> game I built in college which was a long long long time ago.  I usually
> crashed after one or two times around the figure 8.
> ----
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